Ube TUniversftg of Chicago 


A Method of Photographing the Disintegra¬ 
tion of an Atom, and a New Type 

of Rays 


A DISSERTATION 


SUBMITTED TO THE FACULTY OF THE OGDEN GRADUATE SCHOOL 
OF SCIENCE IN CANDIDACY FOR THE DEGREE OF 
DOCTOR OF PHILOSOPHY 


DEPARTMENT OF CHEMISTRY 




BY 

ROGER WILLIAM RYAN 


Private Edition Distributed by 
The University of Chicago Libraries 
Chicago, Illinois 

f Reprinted from the Journal of the American Chemical Society, 
Vol. XLV. No. 9. vSeptember, 1923.] 







Ebe TOniversit]? of (Xbicaflo 


A Method of Photographing the Disintegra¬ 
tion of an Atom, and a New Type 



A DISSERTATION 


SUBMITTED TO THE FACULTY OF THE OGDEN GRADUATE SCHOOL 
OF SCIENCE IN CANDIDACY FOR THE DEGREE OF 
DOCTOR OF PHILOSOPHY 


DEPARTMENT OF CHEMISTRY 


BY 


ROGER WILLIAM RYAN 

H 


Private Edition Distributed by 
The University of Chicago Libraries 
Chicago, Illinois 

[ Reprinted from the Journal of the American Chemical Society, 
Vol. XLV. No. 9. September, 1923.] 





GLCnzi 

•"R^S 

m3 - 


gift 

UNIVERSITY 

NOV 3 


• •• 


♦ 

I « 

V 



A METHOD FOR PHOTOGRAPHING THE DISINTEGRATION OF 
AN ATOM, AND A NEW TYPE OF RAYS 

Introduction 

In 1915 Harkins and Wilson 1 published the first definite theory of the 
composition of the nuclei of atoms in terms of hydrogen and helium. 
1 he theory predicted a general difference of stability between the atoms 
of elements of even and those of odd atomic number. That this pre¬ 
diction is definitely confirmed was shown in a later paper, 2 where it was 
demonstrated that (1) the elements of even atomic number are represented 
in the meteorites by <0 times as many atoms as those of odd number; 
(2) every one of the 7 most abundant elements in the meteorites has an 
even atomic number; (3) every one of the 5 undiscovered elements has an 
odd atomic number; and (4) there are many more atomic species (isotopes) 
of even than of odd atomic number. 3 

An altogether different type of confirmation was supplied later by 
Rutherford. 4 He was able to disintegrate atoms of the elements 5, 7, 
9, 11, 13 and 15 in such a way that they give off hydrogen in every case, 
while he did not obtain any evidence that atoms of elements of even atomic 
number disintegrate at all. Now, the theory of Harkins specifically 
pointed out that the energy of combination of hydrogen to form helium 
is so great that a disintegration of any atom built up entirely of helium 
would probably in no case give hydrogen (Table I), but it indicated that 
elements of odd atomic number contain hydrogen not combined into 

1 Harkins and Wilson, Proc. Nat. Acad. Set., 1, 276 (1915). J. Am. Chem Soc 
37, 1367 (1915). 

2 Harkins, J. Am. Chem. Soc., 39, 856 (1917). 

3 That this follows from the theory of Harkins was shown by N. F. Hall [Ibid 
39, 1606 (1917)]. 

4 Rutherford, Phil. Mag., 37, 538 (1919). Rutherford and Chadwick, ibid 42, 
809 (1921). 


4 


helium, and these would thus be capable of disintegration to give hydro¬ 
gen, 5 as accords with the experimental results. 

Table I 

Energy of Alpha-Particles 


Energy of formation of 1 

g.-atom of helium 

from 4 g.-atoms of hydrogen = —2.8 

10 19 

ergs. (—4.6 X 10 -5 

ergs per atom of He) 




Kinetic energy, 


Source of a-particle 

Velocity 

Per particle X 10 5 Per gram atom X 10 13 

Po 

0.0523 c 

0.812 

4.92 

Ra C' 

.0641 

1.218 

7.38 

Th C 

.0572 

0.970 

5.88 

Th C' 

.0688 

1.404 

8.51 


The table indicates that the highest kinetic energy for any of the a-particles listed 
is less than 1 / 3 the energy of formation of an a-particle from protons and electrons. 

"c” is the velocity of light. 

The scintillation method, as used in these experiments by Rutherford, 
is capable of detecting disintegration particles which have only a range 
greater than 30 cm. in air, since hydrogen atoms which are merely re¬ 
leased from chemical combination in water, hydrogen, and other com¬ 
pounds, have nearly this range, while H + particles (protons) released from 
the nucleus of an atom have a greater range. In the case of the aluminum 
nucleus, for example, it is 90 cm. However, other disintegrations may oc¬ 
cur. Thus, when bombarded by swift a-particles, certain atom nuclei may 
give disintegration fragments, such as a-particles, H + particles, or electrons, 
of a shorter range than 30 cm. Since all such particles, of any range 
down to less than a millimeter, leave visible tracks in the ray track appa¬ 
ratus of C. T. R. Wilson, 6 it would seem that a method could be devised 
which would utilize this apparatus for the detection and study of nuclear 
disintegrations induced by the impact of swift a-particles, provided the 
characteristics which such a disintegration would exhibit could be deter¬ 
mined, and to do this is very simple. 

Photographs of Atomic Collisions. (Collisions of the Nuclei of Atoms) 

In order to disintegrate an atom nucleus artificially it is necessary to 
bombard it with a high-speed helium nucleus (a-particle). 7 The scintilla¬ 
tion experiments of Rutherford, 8 and of Geiger and Marsden 9 have shown 
that the a-particle must pass through an extremely great number of atoms 

5 In the 1915 paper from this Laboratory it was pointed out that the packing effect 
for hydrogen nuclei in a complex nucleus is smaller in all probability when the hydrogen 
is not combined in an a-particle. The atomic weight of nitrogen, 14.01, indicates that 
this is true in the nitrogen nucleus unless the 0.01 in excess over a whole number is due 
to experimental error. 

6 Wilson, Proc. Roy. Soc., 87, 277 (1912). 

7 It is of course possible that high-speed electrons may also induce disintegration. 

8 Rutherford, Phil. Mag., 21, 669 (1911). 

9 Geiger and Marsden, Proc. Roy. Soc., 82A, 495 (1909). 


5 


in order to secure a single collision with the nucleus of an atom, and the 

photographs described in the present paper demonstrate this in a much 
more direct w^ay. 

The writers have taken 21,000 photographs of a-ray tracks in air and a 
few in helium and ethyl chloride. Of these 10,000 were secured by using 
polonium, and 11,000 by using thorium C and C' as the source of the 
a-particles. In all, about 80,000 tracks have been photographed. In 
each of these the a-partiele passes directly through between 100,000 and 
200,000 atoms, provided atoms have the dimensions usually attributed 
to them. In all about 12 billion atoms have been shot through, with the 
remarkable result that in only 3 cases has the nucleus of an atom of the 



Fig. 1.—Sharp atomic collision with a visible 
track of the bombarded nitrogen or oxygen nu¬ 
cleus. The fork exhibits conservation of mo¬ 
mentum which proves that the nucleus remains 
stable under the impact 



Fig. 2.—Atomic collision. Shows 
an apparent but not a real, contra¬ 
diction of the principle of conserva¬ 
tion of momentum 


gas been hit sharply enough to give a retrograde motion to the a-particle 
after the collision. Thus for each sharp collision the a-partieles have 
had to shoot through about 4 billion atoms. This fact taken alone would 
indicate that the radius of the nucleus of an atom of air 10 is of the order 

°f that of the atom, w r hich would give the nucleus a radius of the 

order of a little more than 10 -13 cm. Since the number of hits is so small, 
a closer estimate could be secured by a consideration of the numerous cases 
in which the particle is slightly deflected from its path by the repulsion 
between its positive charge 11 of 2 and that of 7 on the nitrogen nucleus. 
Fig. 1 gives a reproduction of a photograph of the sharpest nuclear 

10 An atom of nitrogen or oxygen. 

11 In this paper the charge on the electron is taken as of unit magnitude. 
















6 


collision ever obtained, with one exception. The a-partiele from a po¬ 
lonium source, with an initial velocity of 1.357 X 10 10 cm. per second, is 
turned through an angle of 155° and rebounds for a distance whose hori¬ 
zontal projection is about 5.2 mm. while the nitrogen nucleus is projected 
forward for a corresponding distance of 3.8 mm. If the track of this 

nucleus is projected backward it 
may be seen that the line thus 
obtained lies slightly closer to the 
track of the on-coming, than to 
that of the retreating a-particle. 
This is essential in order that 
the impact shall exhibit conser¬ 
vation of momentum, since the 
a-partiele has much less momen¬ 
tum after the collision than it had 
before. 

The characteristics of an ordi¬ 
nary collision are: (1) the initial 
track splits into two branches— 
that is, three lines converge at 
one point in space, (2) momentum is conserved in the collision, and 
(3) the three tracks lie in one plane. In addition, energy is conserved, 
provided the presumably small 
amount of energy radiated is 
taken into account. 

Fig. 2 shows the third sharpest 
collision photographed, and is 
presented as an example of an ap¬ 
parent, but not a real, contradic¬ 
tion to the principle of conserva¬ 
tion of momentum.in an impact. 

Here the track of the bombarded 
nucleus does not appear in the 
photograph. Presumably this is 
due to a lack of water vapor for 
the production of the track. This 
often occurs and is due to a con¬ 
version of the water vapor in a particular region into the minute drops 
which form a track. These drops are electrically charged and are drawn 
out of the gas by an electrical field of 500 volts per cm. If a second track 
passes through this region within a few tenths of a second there is not 
sufficient moisture to enable water to gather in drops on the ions, so there 
appears to be a break in the track. In one photograph (Fig. 3) there 



Fig. 4.—Loop formed by two a-ray tracks, 
showing old track threading the loop 



Fig. 3.—Effect of old tracks in the taking up 
of water vapor 









7 


are two such breaks in a transverse track; one due to a track which has 
passed so recently that it appears very sharp, while the other is caused 
by an older track which has become diffuse and has been mostly swept 
out by the electric field. The transverse track was photographed almost 
immediately after its formation. If it had remained slightly longer, ions 
produced by the a-particle would have diffused out (or have been pulled 
out by the field) of the dry region into a space which contains more 
moisture, and here an apparent loop somewhat like half of an ellipse cut 
along its longer axis would have appeared in the track. This bends either 
upward or downward corresponding to whether the new track lies above 
or below the old one. In some cases it bends in both directions, and the 
remnant of the old track is photographed as if threading the eye of a 
needle. (Fig. 4.) 

Characteristics Exhibited by a Photograph of the Disintegration of an 

Atom 

lu order to secure the transfer of a large amount of energy to the nucleus 
to be disintegrated, the incident a-partiele should have as high a speed 
as is possible, and also the collision should be as sharp as can be obtained. 
The former of these conditions was met in the experiments under dis¬ 
cussion in this section, by the use of thorium C and C' as a source. Fig. 
5 shows the sharpest collision ever photographed. The angle in space 
between the lines representing 
the tracks of the ^-particle is 
only 15°, so that the particle 
has been turned through an 
angle of 165°. 

Where a nuclear collision oc¬ 
curs, the initial track splits into 
two branches. If it should split 
into three branches, one of these 
would be due to the rebounding 
a-particle, one to the forward 
path of the nucleus which is hit, 
and a third track, provided it Fig. 5.—Sharpest collision of atomic nuclei 
were due to a positively charged photographed. Shows the number of tracks 
nucleus, would indicate a par- characteristic of a n atomic disintegration 

tide torn off from the bombarded nucleus by a process of disintegration. 
Now it happens that the tracks of positively charged nuclei are usually 
easily distinguished from those of negative electrons, since the latter are 
much fainter, and more highly curved. It might seem that the extra track 
was caused by the collision of the a-particle with a second nucleus which is 
not very distant from the first one, but nuclear collisions are so rare that this 






8 


is extremely improbable. Even if it should occur, the momentum and en¬ 
ergy relations are such as to give a type of double fork which could probably 
be recognized. Thus, the energy imparted to the nuclei would be taken 
from that of the a-particle, so the latter should have a shorter range if 
it gives energy to a second nucleus than if it hits only one. The energy 
difference can be estimated, within rather wide limits, from the range of 
the nuclei that are hit. Even at such high speeds as those used in this 
work there should be no considerable departure from the principle of 
conservation of momentum in the combined impact and disintegration. 
Therefore, whenever all of the tracks show, the photograph should ex¬ 
hibit this conservation. However, Fig. 2 demonstrates that the track of 
even a highly charged positive nucleus may fail to appear. From this point 
of view it would not seem strange if the faint tracks due to high speed 
electrons or H + particles should remain invisible. 

If a disintegration should occur, the a:-particle and the bombarded 
nucleus alone would not be expected to exhibit conservation of momentum, 
though momentum is always conserved in a collision unaccompanied by 
a disintegration. This indicates that if the tracks of these particles as 
shown by the photograph are such that momentum is not conserved 
for the two particles alone, an atomic disintegration has taken place. 

In a simple collision it is to be expected that all of the tracks should lie 
in a plane. If a disintegration occurs this should not be true except by 
accident. 

Fig. 5 shows an a-ray track which splits into 3 branches after the col¬ 
lision occurs, so it has this characteristic to be expected if the bombarded 
nucleus disintegrates. The extra track is too bright to be caused by an 
electron, though its extreme brightness is due to the fact that it is photo¬ 
graphed almost “head-on.” It is evident that the visible tracks do not 
exhibit conservation of momentum, which may indicate that other par¬ 
ticles are emitted at the same time, but do not show. The tracks in the 
original negative are sharp—much sharper than in the reproduction,— 
and a study of the film under the microscope seems to show that if the 
third particle is neglected, the remaining tracks do not show conservation 
of momentum as they should if the collision is a simple one. The question 
arises, could the extra track have been formed by the disintegration of 
a radioactive particle exactly at the point of collision at almost exactly 
the time of the collision? It cannot be stated that this is an absolute 
impossibility, but it is certain that the probability of such a coincidence 
is excessively small. At any rate the method used in these experiments 
will in general photograph atomic disintegrations if they occur. 

Zeta ($*) Rays 

Delta(5) -rays were first found in gases by Bumstead, who gave them 
a sufficient range for observation by causing them to appear in hydrogen 


9 



at low pressures. The rays are given off at nearly right angles to the 
track of the a-ray which produces them. They are very short in air at 
ordinary pressure, not more than 0.5 mm. in length, and end in a knob 
which is easily visible. It is supposed that they are produced by low speed 
electrons removed from the atoms by the passage of the particle. 
C. T. R. Wilson has secured 
beautiful photographs of these 
rays, and the writers have often 
observed them, particularly in 
helium. 

Fig. 6 shows a new type of 
rays, which may be called zeta 
(f) rays. Their range is very 
many times greater than that of 
the 5-rays. They are much 
rarer, and give faint, but very 
definite tracks. Their faint¬ 
ness and high curvature makes 

it seem probable that they are Probab i y due t0 electrons 
due to electrons torn out of the 

atoms, possibly from the K level. The figure indicates that the ejection 
of the first ^-ray does not materially change the direction of the a -ray 
track, and this, taken in connection with the length of the tracks, shows 
that the mass of these particles must be low, as would be the case if they 
are electrons. 

These rays present features of extreme interest. 12 Thus in Fig. 6 the 


Fig. 6.—Rays of a new type (Zeta Rays). 


12 In a letter in Nature, 111, 463 (April 7, 1923), Bose and Ghosh present three 
interesting photographs of a-ray tracks in helium. The interpretation of these photo¬ 
graphs is difficult, since only one projection was obtained, but it seems that electrons 
have been ejected from helium atoms by the passage near or through them of a-par- 
ticles. In this respect there is some similarity to what has been discussed above under 
the designation of f-rays. However, their rays possess quite different characteristics, 
since they are not projected backward. In order to determine the nature of the rays 
secured by them two projections should be obtained, since one projection may reveal 
entirely different characteristics from those exhibited by the other. 

Their photographs do not resemble those which depict the collision of 2 atom 
nuclei, since in each case there seems to be a continuation in a straight line of the initial 
a-ray track beyond the point where the branching occurs. 

Since the nucleus of the argon atom is larger, and the electric field around it is also 
stronger, than for nitrogen or oxygen, it is much easier to secure photographs of atomic 
collisions in argon than in either of the two other gases. Blackett [ Proc . Roy. Soc., 
103A, 62 (1923)] obtained one collision in which the a-particle from polonium was 
turned through an angle of about 110°, but this is much less sharp than those given in 
Fig. 1 (125°) and in Fig. 3 (165°). 

In addition to the 21,000 photographs of a-ray tracks in air described in the body 
of the paper, the writers have now obtained about 20,000 photographs of tracks in argon. 




10 


2 ray-tracks are practically parallel. Both start upward and give curves 
that are convex upward. The particles have a considerable retrograde 
motion, and their range is about 3 mm. 




Of these, two are so remarkable that they are presented in Figs. 7 and 8. One of these. 
Fig. 7, represents by far the most remarkable simple collision yet obtained, in that the 

speed of the a-particle is far higher 
than for any previous case. Thus, 
even after the loss of a considerable 
fraction of its energy by collision with 
an argon nucleus, the velocity of the 
rebounding a-particle is nearly V 20 that 
of light, or 18,000 times that of the 
fastest rifle bullet. The range of the 
argon nucleus in argon is very high, 
about 8 or 9 mm. under the conditions 
of the experiment. The most remark¬ 
able feature of this photograph is that 
it shows that even under this terrific 
impact the argon nucleus remains in- 
Fig. 7.—Collision of fast a-particle with the ^act, so its stability must be of an ex¬ 
nucleus of an argon atom. The hardest hit t rem dy high order. The a-particle in 
thus far photographed. The fork exhibits Question has its source in thorium C , 
conservation of momentum, which proves that so *ts initial velocity is 0.688 c. 
the argon nucleus has not disintegrated ^ nuclear collision which is not 

simple is represented in Fig. 8. An 
a-particle from a source consisting of thorium C and thorium C' strikes an argon 
nucleus and is deflected diagonally upward through more than 45°. Two other tracks 
spring from the point of collision, as is 
made evident by the two projections 
which go more or less downward. As 
nearly as can be told, there is a total 
conservation of momentum in the im¬ 
pact. The appearance of the two 
tracks for the bombardment nucleus 
instead of one indicates that (1) the 
argon nucleus disintegrates, or (2) the 
bombarded argon nucleus collides 
with a second argon nucleus within 
0.5 mm. of its starting point. There 
is a third possibility, that the a-par¬ 
ticle hits 2 argon nuclei in direct suc¬ 
cession, but the probability of such 
an occurrence is almost negligible. 

The angle between the two tracks 
under discussion is about 80°. From 
the 20,000 photographs of a-ray 
tracks in argon, and by a considera¬ 
tion of the stopping power of argon, 
it has been calculated that the probability that an argon nucleus will hit a second argon 
nucleus within a distance of 0.5 mm. in such a way as to give an angle as great as 80 a 
between them, is small, possibly about 1 in 1000, the most uncertain factor being that 


Fig. 8.—Double collision of atom nuclei in 
argon. The a-particle collides with the nucleus 
of an argon atom, and probably this in turn col¬ 
lides with a second argon nucleus. Both views 
of the collision are clear in the original photo¬ 
graphs 


1 







11 


Experimental Part 

Ihe apparatus used is shown in Figs. 9, 10, and 11. 

It is essentially a modified Shimizu-Wilson apparatus,’ 3 and consists of a glass 
c oud chamber N (Fig. 9) fitted with a piston and provided with a means for securing 
a sudden expansion. The roof of this cloud chamber is a glass plate K coated under¬ 
neath with moist gelatin to which copper sulfate has been added. The top of the 
piston is also coated with moist gelatin blackened with india ink (not waterproof) 
A source of a-rays is provided at one side of the chamber as shown in Fig. 11 (B). Elec¬ 
trons given off and ions formed by the passage of ^-particles through a gas serve as nu¬ 
clei for the formation of water drops during the expansion and consequent cooling of the 
nearly saturated gas. A motion picture camera A is provided with a driving mechanism 
and an optical system such as to take views at right angles at the end of each expansion. 

The most important modifications of the original Shimizu-Wilson Appa¬ 
ratus and the method of securing the rays are: (1) the use of a standard 



motion picture camera driven by a flexible shaft (A, Figs. 9 and 10); 
(2) a special cam R (Fig. 10) used to secure a sudden expansion (a 

related to the speed of the argon nucleus. Nevertheless, the characteristics of the tracks 
around the point of impact suggest that this double collision is probably what has 
occurred. The shortness of the two tracks, and their approximate equality in length 
seem to point to the conclusion that in this case a disintegration has not occurred. 
That the two short prongs are not due to electrons is indicated by the fact that the <x- 
particle is deflected too sharply to give conservation of momentum with such light par¬ 
ticles. Thus it seems probable that Fig. 8 represents the first “double collision” of atom 
nuclei to be photographed, though the possibility that the argon nucleus has disinte¬ 
grated is not at all excluded. (Received August 9, 1923.) 

13 Shimizu, Proc. Roy. Soc. (London), 99A, 425 (1921). 









































































































12 


» 


somewhat similar device is used by Blackett 14 ); (3) the use of ThC, 
ThC', and RaC as source of a-rays; (4) the electric field used to sweep 
out water drops between the expansions is applied by means of brush con¬ 
tacts S instead of by a commutator; (5) a 
variable voltage transformer and a 500 
volt kenotron (with a lmf. condenser) 
is used as a source of this field; (6) the 
use of a moving screen prevents a-particles 
from entering the chamber except during 
maximum expansion. 

The cylinder of the cloud chamber was 
ground from a well annealed borosilicate 
glass blank so as to give the uniform small 
clearance necessary to secure good cloud 
tracks. A capillary stopcock was pro¬ 
vided on one side so that various gases 
might be introduced. Fig. 11 shows the 
means of screening a-particles from the 
chamber except at maximum expansion. 
A is a copper strip used as a screen to 
prevent rays from entering the chamber 
except at the time of maximum expansion 
and B is a Bakelite block holding the copper wire carrying the active 
deposit. This is so bored that a-particles are able to shoot out in only 
a narrow beam; a-particles shot out at 
other times than at maximum expansion 
give diffuse tracks. 

ThC and ThC' served as the source 
of the a-particles for much of this work. 

About 1 mg. of radiothorium is first 
dissolved in coned, hydrochloric acid. 

Then a copper wire is coated with wax, 
except at one end, and immersed in this 
solution for about half an hour or longer 
if the solution is weaker. The “active 
deposit” of thorium, consisting largely 
of ThC and ThC', deposits (by displace¬ 
ment) on the exposed copper. The end 
of the wire is then cut off and placed at 
the base of the Bakelite block (B, Fig. 11). The very short life of ThC 
(only a few hours) makes it necessary to prepare this source immediately 
before use. Polonium sources are prepared in a similar way from solu- 

14 Blackett, Proc. Roy. Soc., 102A, 294 (1922). 



& 

7 ' 

h 



% 

£ 

cn 

Lii 




Fig. 11.—Diagram of ionization 
chamber 



apparatus 



/ 









































































13 


tions of RaF in hydrochloric acid but have a life measured in months 
instead of hours. 

The photographic equipment includes a motion picture camera (A, Fig. 
9) driven by a flexible shaft which carries a clutch D and thus allows the 
timing of the shutter with the expansion. This camera is provided with 
a Taylor-Hobson-Cooke F/2 lens in a focusing mount G, and an optical 
system which consists of two adjustable surface silvered mirrors E, and a 
prism F. These mirrors are carried on a 3-point support that slides along 

a track shown in Fig. 9, while the prism is clamped directly below the 
lens. 

The cloud chamber is illuminated by two d. c. right-angle arcs, each 
provided with an 11cm. aspheric condenser and a cooling cell. Am¬ 
meters are used in both circuits and the current is adjusted so as to afford 
uniform illumination. Slits are provided on the cooling cell J and at Y 
in order to prevent the beam of light from striking the lower surface of the 
glass roof of the cloud chamber or the top of the piston at the moment of 
expansion. 

In both views parts of the frame, the driving apparatus, etc., have been 
left out so that a clearer view of the essential parts of the apparatus could 
be given. 

For a successful photograph a very sharp expansion is necessary, so that 
a fairly heavy spring O is used to force down the piston immediately upon 
its release by the cam R (Fig. 10). Means are provided for varying the 
length and the height of the stroke at U, and a locknut just below P regu¬ 
lates the expansion ratio. Somewhat exact adjustment is required to give 
the optimum conditions. A monatomic gas such as helium or argon re¬ 
quires a small expansion to secure the necessary cooling; air (nitrogen 
and oxygen) and other diatomic gases require a somewhat greater expan¬ 
sion, and a polyatomic gas such as ethyl chloride requires so great an ex¬ 
pansion that it is difficult to prevent leakage into the apparatus. In the 
latter case the leather gasket of the piston must also be lubricated with 
some material not soluble in ethyl chloride (as glycerol and graphite). 

The writers wish to thank the Visual Education Society for the use of 
one of their motion picture cameras; the Gibb’s Fund of the National 
Academy of Sciences for a grant partly used in this work; Dr. H. N. McCoy 
for the polonium used and for the loan of radium; Dr. H. S. Miner of the 
Welsbach Company, for the loan of mesothorium and radiothorium; Mr. 
Henry Burke, of Burke and James, Chicago, for aid in securing the remark¬ 
ably fine lens used; and Mr. Paul L. Gross for assistance in the photo¬ 
graphic work. 

Summary 

1. The rarity of a collision between a fast helium nucleus (a-particle) 
and the nucleus of an atom in a gas through which it is passing, increases 


14 


greatly as the directness of the collision increases. At the time of the be¬ 
ginning of the experimental work described in the present paper no photo¬ 
graph of a collision had been obtained sufficiently sharp to give the helium 
nucleus (a-particle) a retrograde motion after the collision. Thus far 
photographs have been taken of enough tracks so that the a-particles 
have passed through about 12 billion atoms in air, with the result that 
in only 3 cases has such a rebound been obtained. This alone would indi¬ 
cate that the nucleus of an atom of nitrogen or oxygen is of the order of 
slightly more than 10~ 13 cm. in radius. A more accurate value can be 
obtained by a mathematical analysis of all of the deflections of the a- 
particle through smaller angles. 

2. In an ordinary collision 3 tracks meet in a point: one for the a- 
particle before, and a second for the same particle after the collision. If 
the bombarded nitrogen or oxygen nucleus should disintegrate, then at 
least 4 tracks should meet, the additional track being due to a fragment, 
such as an electron, a hydrogen or a helium nucleus, disrupted from the 
bombarded nucleus. The chance of such a disintegration increases rapidly 
with the directness of the collision, and with the speed of the a-partiele. 
For some unknown reasons the tracks of high-speed a-partieles have not 
been photographed in previous work. While using such high-speed par¬ 
ticles the writers secured a remarkable photograph of by far the most 
direct nuclear collision recorded in this way up to the present time, with 
the result that a fourth track, which should characterize an atomic dis¬ 
integration, appeared in the photograph. That this track springs from 
the proper point in space is shown by the two projections obtained simul¬ 
taneously. These give two views at an angle of 90°. Whatever may be 
the final decision with respect to this individual photograph, the method 
presented in this paper will reveal atomic disintegrations to give either 
helium or hydrogen or electrons, provided they occur during the operation 
of the apparatus, though the labor and expenditure of funds involved in ob¬ 
taining such photographs may prove to be very great. 

3. Rays of a new type, designated by the writers as zeta (f) rays, 
are shown in Fig. 6. Here the a-particle evidently drives particles from 
2 widely separated atoms in its path. It is remarkable that the two tracks 
thus obtained lie in almost parallel planes, both are highly curved and 
almost parallel lines, and both have a sharp retrograde motion. It seems 
probable that the f-rays are due to electron emission, but this is not 
certain. The particles must be very light, however, since the direction 
of the a-particle is not materially affected by the emission, and the tracks 
of the particles are moderately long, very much longer than those of the 
previously recorded 5-rays. 

4. Thus far, 40,000 photographs have been secured, and 3,000 photo¬ 
graphs are obtained for each additional hour of operation of the apparatus 


15 


with photographic attachments. A number of collisions have been ob¬ 
served visually during periods when the camera is not in use. 

5. The method presented in this paper is being used to test experi¬ 
mentally the stability of atom nuclei. Such tests have not been made 
efore. Rutherford’s experiments, for example, reveal the disintegration 
o an extreme!} minute number of atoms to give long range hydrogen par¬ 
ticles, but they do not show whether or not a- or any other short range 
particles are emitted. The difficulty of the present method lies in the small 

number of direct hits obtained. Less direct impacts are relatively nu- 
merous. J 

The remarkable feature of the present work is that in no case has any 
one of these oblique impacts effected a disintegration of the nucleus 
Even more remarkable is the fact that the argon nucleus in Fig. 7 remains 
intact even under the sharp impact of a helium nucleus from Thorium 
C (a-particle) with a velocity of 25,000 or 30,000 times that of the swiftest 
rifle bullet, immediately before impact. This is evidenced by the fact 
that the visible tracks around the point of collision exhibit conservation 
of momentum. Fig. 1 illustrates the stability of an atom of air (nitrogen 
or oxygen) in exactly the same way except that the velocity of the a- 
particle just before impact is not quite so high. 

It may be added that the photographs chosen for this paper have been 
selected altogether from the standpoint of the importance of the events 
they represent, even although they may not be so good as some of the 
others from the purely photographic viewpoint. 


* 


« 






I 









/ 



X 
















\ 













. • 


' 




-X 


✓ 





\ 



/ 














* 





/ 


{ 





i 

















LIBRARY OF CONGRESS 



0 003 750 113 A 


















































